Individual strings of conducting carbon cones and discs in a polymer matrix: Electric field‐induced alignment and their use as a strain sensor

We demonstrate micromechanical strain sensors with integrated readout based on carbon nanocones and discs (CNCs) which are aligned into a string‐like formation using an alternating electric field and studied by AC impedance spectroscopy and electromechanical methods. The CNC particles are first dispersed into a polymer matrix with a particle fraction of 0.1 vol %. This value is well below the percolation threshold (～ 2 vol %), which suppresses particle aggregation and facilitates transparency allowing the use of an UV‐curable polymer. Alignment was carried out with a 1 kHz, 4 kV/cm electric field and is a consequence of dielectrophoretic effect. It develops in minutes and makes the initially insulating, nonaligned material conductive. This is followed by UV curing of the polymer matrix, which renders a solid state device. The stretching of the aligned strings in the cured polymer leads to a reversible piezoresistive effect, and a gauge factor of about 50 is observed. This is in a sharp contrast to CNC films with particle fraction above percolation threshold (13 vol %), which are conductive but not sensitive to stretching. The strings are Ohmic in nature and moreover show higher DC conductivity (22–500 S/m) compared to identically prepared carbon black strings (1–22 S/m). © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Controlling supramolecular polymerization through multicomponent self‐assembly

The self‐assembly into supramolecular polymers is a process driven by reversible non‐covalent interactions between monomers, and gives access to materials applications incorporating mechanical, biological, optical or electronic functionalities. Compared to the achievements in precision polymer synthesis via living and controlled covalent polymerization processes, supramolecular chemists have only just learned how to developed strategies that allow similar control over polymer length, (co)monomer sequence and morphology (random, alternating or blocked ordering). This highlight article discusses the unique opportunities that arise when coassembling multicomponent supramolecular polymers, and focusses on four strategies in order to control the polymer architecture, size, stability and its stimuli‐responsive properties: (1) end‐capping of supramolecular polymers, (2) biomimetic templated polymerization, (3) controlled selectivity and reactivity in supramolecular copolymerization, and (4) living supramolecular polymerization. In contrast to the traditional focus on equilibrium systems, our emphasis is also on the manipulation of self‐assembly kinetics of synthetic supramolecular systems. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 34–78     

pH reversible micelle transition of a tertiary amine-based gemini surfactant in aqueous solution

In this work, we prepared a tertiary amide-based gemini surfactant (DSTAPA), which contained two pH-sensitive tertiary amide head groups. Then the molecule state distribution and self-assembly transition of the surfactant in aqueous solution were investigated under different pH conditions. The DSTAPA molecules were on the states of double cationic (DSTAPAH²⁺), single cationic (DSTAPAH⁺), and double tertiary amine groups (DSTAPA) under acidic, neutral, and basic conditions, respectively. With the variation of the molecule states, the sample was water-like below pH of 6.8 and immediately transformed to gel-like fluid between pH of 6.8 and 7.8, then changed to white precipitate with the further increase of pH value. Furthermore, the microstructure and regulation mechanism were investigated by rheological measurements, dynamic light scattering, and cryogenic transmission electron microscopy. The appearance and micelle transitions of the DSTAPA aqueous solution are actually owing to the spherical–worm-like micelle transition, leading to dramatic viscosity increase and hydrogel formation. This transition was completely reversible and repeated for at least three cycles. Finally, a reasonable mechanism of the transition was proposed based on the viewpoints of the molecular states and micelle structures. The DSTAPA aqueous system with pH-reversible property has a great potential application in oil and gas production.     

Synthesis and solvent‐dependent micellization of the amphiphilic block copolymer poly(styreneboronic acid)‐block‐polystyrene

The amphiphilic organoboron block copolymer poly (styreneboronic acid)‐block‐polystyrene ( PSBA‐b‐PS ) has been prepared through a postpolymerization modification route from the silicon‐functionalized block copolymer poly(4‐trimethylsilylstyrene)‐block‐polystyrene ( PSSi‐b‐PS ). PSBA‐b‐PS is obtained through highly selective reaction of PSSi‐b‐PS with BBr₃ at room temperature and subsequent hydrolysis of the BBr₂‐functionalized intermediate. Transmission electron microscopy studies demonstrate that PSBA‐b‐PS undergoes pH dependent micellization in aqueous solution. Different morphologies could be realized by using different mixtures of water and organic solvents. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2438–2445, 2010     

Characterization of patterns in rimming flow

Patterns were generated inside a horizontal cylinder rotating at low speeds. The cylinder was filled with a very low volume liquid fraction of 1.8% of Newtonian fluid and the rotation speed ranged between 0.08 and 5.2 s⁻¹. A novel laser-plane technique was utilized to obtain time series from each pattern. This enabled the characterization of fluid patterns using Fourier spectral (FS) and dynamical-systems (chaotic) techniques such as the recurrence map, correlation dimension (D₂) and Hurst exponent (H). Four patterns were found (fingers, furrows, waterfall and smooth tooth) before annular flow was reached. The results indicate that the FS technique not is suitable for flow pattern characterization; and H only has the ability to indicate a possible pattern change. The best tool for indicating the pattern transitions and the inner coat liquid evolution was found to be recurrence maps and D₂.     

Fast Gaussian wavepacket transforms and Gaussian beams for the Schrödinger equation

This paper introduces a wavepacket-transform-based Gaussian beam method for solving the Schrödinger equation. We focus on addressing two computational issues of the Gaussian beam method: how to generate a Gaussian beam representation for general initial conditions and how to perform long time propagation for any finite period of time. To address the first question, we introduce fast Gaussian wavepacket transforms and develop on top of them an efficient initialization algorithm for general initial conditions. Based on this new initialization algorithm, we address the second question by reinitializing the beam representation when the beams become too wide. Numerical examples in one, two, and three dimensions demonstrate the efficiency and accuracy of the proposed algorithms. The methodology can be readily generalized to deal with other semi-classical quantum mechanical problems.     

Polynomial factorization over

We describe algorithms for polynomial factorization over the binary field , and their implementation. They allow polynomials of degree up to to be factored in about one day of CPU time, distributing the work on two processors.

     

Heteronuclear decoupling under fast MAS by a rotor-synchronized Hahn-echo pulse train

A new heteronuclear decoupling mechanism under fast magic-angle spinning MAS is introduced. It is based on refocusing the coherences responsible for the dephase of low-gamma nuclei ((13)C, (15)N) transverse spin-polarization in the presence of strongly dipolar-coupled protons, and has the advantage that can be implemented by pulsed techniques, with all the benefits resulting from a reduced duty cycle compared with conventional decoupling by continuous rf irradiation. The decoupling efficiency of a simple rotor-synchronized Hahn-echo pulse train is analyzed both theoretically and experimentally. It was found that a substantial improvement in sensitivity and resolution can be achieved in compounds with small (1)H chemical shielding parameters even at moderate sample spinning, and some interesting applications are shown. It is also shown that much faster spinning frequencies, or alternative refocusing sequences, are needed for applications on rigid organic solids, i.e., in systems with larger (1)H chemical shifts.     

Spin-lattice relaxation and a fast T1-map acquisition method in MRI with transient-state magnetization

The magnetization under the spin-lattice relaxation and the nuclear magnetic resonance radiofrequency (RF) pulses is calculated for a signal RF pulse train and for a sequence of multiple RF pulse-trains. It is assumed that the transverse magnetization is zero when each RF pulse is applied. The result expressions can be grouped into two terms: a decay term, which is proportional to the initial magnetization M0, and a recovery term, which has no M0 dependence but strongly depends on the spin-lattice relaxation and the equilibrium magnetization Meq. In magnetic resonance pulse sequences using magnetization in transient state, the recovery term produces artifacts and can seriously degrade the function of the preparation sequence for slice selection, contrast weighting, phase encoding, etc. This work shows that the detrimental effect can be removed by signal averaging in an eliminative fashion. A novel fast data acquisition method for constructing the spin-lattice relaxation (T1) map is introduced. The method has two features: (i) By using eliminative averaging, the curve to fit the T1 value is a decay exponential function rather than a recovery one as in conventional techniques; therefore, the measurement of Meq is not required and the result is less susceptible to the accuracy of the inversion RF pulse. (ii) The decay exponential curve is sampled by using a sequence of multiple pulse-trains. An image is reconstructed from each train and represents a sample point of the curve. Hence a single imaging sequence can yield multiple sample points needed for fitting the T1 value in contrast to conventional techniques that require repeating the imaging sequence for various delay values but obtain only one sample point from each repetition.     

NMR-microscopy with TrueFISP at 11.75T

The purpose of this paper is to demonstrate that a fully balanced gradient echo technique (TrueFISP) can be used for microscopic experiments at high static magnetic field strengths. TrueFISP experiments were successfully performed on homogeneous and inhomogeneous objects at 11.75T. High-resolution TrueFISP images were obtained from phantoms, plants, formalin-fixed samples, and from an isolated beating rat heart with an in-plane resolution of 78 micro m and a slice thickness of 500 micro m. The signal-to-noise ratio (SNR) gain of TrueFISP compared to conventional gradient echo or spin echo sequences will allow faster acquisition times or an improvement in spatial resolution for microscopic experiments.